There are, of course, varying widths but I believe those are the most common. 20x8.5 sounds stupidly skinny for how tall a 20" wheel is. I'd think you'd be hard-pressed to find 35's that are rated for a mere 8.5" width. My 305's (34.3") tires are on 17x8's .... and 8" is the absolute thinnest wheel that's within proper spec for the tires. You WILL have to trim to avoid rubbing if you do a 20x10, though. Expect it. (I had to trim for 17x8's but it was minor.)

I like a thinner rim on a tire. The reason for that is I like the sidewall to protect my investment on the rims. With that set up the tire will stick out just a little past the edge of the rim. So if you were to take a straight edge and run it from one side of the tire's side wall across the rim to the other side of the side wall the edge of the straight edge will not touch the rim. Were as if you go with a wider rim the tire sidewall will be level with the sidewall. So the tire does not protect the rim. I believe the safe number is 2"'s. So say you get a 12" wide tire a 10" wide rim is ok.

These are my 2 trucks; one with a 20x10, one with a 20x8.5. FYI, the overall height of the tires on these trucks is the same as the stock tire height. Well, let's just say very close....within a 1/2" or so.

Most tires these days for larger diameter wheels have lip protectors on them. The lip of the wheel is recessed into the tire regardless of wheel width. My Nittos are this way. An 8.5" wide wheel is also perfectly acceptable for a nothing wider than a 12.50, so a 35 x 12.50 will work just fine on a wheel that width.

If you look along the bottom of the wheel in this pic, you can make out the ledge on the tire that prevents the lip of the wheel from contacting things like curbs.